Surface inspection tool

ABSTRACT

A laser based inspection tool (LIT) for inspecting planar surfaces is described. In a preferred embodiment the LIT can simultaneously inspect both planar surfaces of disks for use in disk drives. In one embodiment of the invention, a disk is moved into an inspection subcompartment between a pair of air knives which blow partially ionized air onto the planar sides of the disk to remove loose particles adhering thereto. After the disk moves through the air knife streams, the two laser beams scan the two sides of the disk. Preferably the scan occurs after the air knives have been turned off and as the disk moves out of the inspection subcompartment. The subcompartment may optionally have an air source which forces air to flow out of the subcompartment to aid in maintaining a clean environment for inspection.

RELATED APPLICATIONS

Commonly assigned, related applications with Ser. Nos. 08/840,354;08/840,351; 08/841,214; 08/841,037; 08/840,352; 840,355 and 08/840,339were filed concurrently with this application.

FIELD OF THE INVENTION

The invention relates to the field of precision surface analysis fordefects. More particularly the invention relates to laser based toolsfor obtaining data on surface features by optical means.

BACKGROUND

Magnetic and optical disks require precision surfaces with extremely lowdefect rates to function properly. A typical magnetic disk comprises asubstrate on which multiple layers of various materials are deposited.For example, an aluminum substrate might be coated electrolessly withNiP then sputtered with thin films of Cr as an undercoat, a cobalt alloymagnetic layer and a hydrogenated carbon overcoat. Depending on thestage of the process these surfaces are not necessarily uniform. Forexample, after the NiP has been applied a small circular band on thesurface of the disk may be textured using a laser to form microscopicbumps. This textured region is intended to provide a low stiction areafor the sliders to rest during nonoperating periods. In addition tointentional variations there may be various types of defects. As thedisks progress through the manufacturing process various tests andinspections are used to detect defective disks so that they may eitherbe reworked or discarded. In addition to visual inspections, a disk maybe subjected to glide tests which are sensitive to the flatness of theplanar surfaces, as well as magnetic read/write tests. Due to highcapacities of magnetic disks it is typically not practical tomagnetically test each bit which can be stored on the disk.

Laser surface inspection of the disks if sufficiently precise mayactually be superior to current magnetic tests in detecting defects.Magnetic defects are usually associated with visible defects, but thevisible defects can be detected more efficiently through laserinspection even though the laser spot size is considerably larger thanthe area in which a bit can be recorded. Thus, laser inspection allowsgreater test coverage of the disk in a cost effective manner.

Various laser inspection devices are known in the art. Commonly assignedU.S. Pat. No. 5,220,617 by Bird, et al. describes a laser scanner forgreen sheets to detect via errors. The sheets are moved on an air trackto a transport table which translates them as the scan occurs. Only oneside is scanned. The system uses a rotating polygon mirror to scan andto capture the reflected light. The bright field reflected light iscaptured at the hole-in-plate splitter and directed to a single fibre.This channel detects contrast between the conductive paste and the greensheet. The dark field light reflected light is captured by fiberslocated near the surface of the object. The incident light isperpendicular, but there is a suggestion that other angles are possible.There is a start of scan mirror adjacent to the object, but its functionis said to be to provide an initialization or start/stop point. Thereference signal is obtained from the initial part of the green sheet.The lens assembly is a flat field telecentric anamorphic f-theta lenssystem. The f-theta condition corrects for the pincushion distortion. Afocusing telescope converges the image down to a slit. The shaping lenssystem results in a collimated bundle 10.8 mm by 130 microns on thepolygon. This shape is said to be selected for pickup by the fiberbundles.

SUMMARY OF THE INVENTION

The inventions will be described as embodied in a laser based inspectiontool (LIT) for simultaneously inspecting both planar surfaces of disksfor use in disk drives. The LIT uses low angle reflected light ratherthan scattered light from the surface to simplify the design, to allowabsolute reflectivity measurements if desired and to aid in thedetection of certain types of disk defects such as stains. Since thesurfaces of the disks are extremely sensitive to physical contact theLIT uses a mechanical lifter which, without clamping or spinning, movesthe disk through the laser scan lines to allow the entire surface oneach side of the disk to be scanned. Inspection or test systems whichrequire the disks to spin are complex and increase the risk of damage tothe disk. The line scanning is performed using a rotating polygonalmirror (scanner) which also captures the beam reflected from the disksurface. A telecentric lens assembly (TLA) acts to ensure that the laserbeam is incident at a constant nearly perpendicular angle as the beamscans across the disk. The TLA is designed to have a very flat fieldcurvature through the scanning line to keep the spot size sufficientlyconstant for accurate detection. A small deviation from perpendicularincidence in the cross-scan direction allows the reflected beam from thedisk to be separated from the incident beam for detection and analysis.The rotational position of the two polygonal scanners are synchronized,but with one being angularly displaced to avoid interference when thebeams scan across the hole in the center of the disk.

The reflected light is digitized and stored in a memory accessible by acomputer. The edges of the disk are detected and a mask is applied todirect the defect detection only to meaningful areas of the disk. Thetechniques for detecting defects include use of a median filter andderivative analysis.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the optical elements in the path from the laser source tothe surface to be inspected.

FIG. 2 shows the optical elements in the path for the reflected laserlight from the surface being inspected.

FIG. 3 shows the optical elements in the path for the reflected laserlight from the calibration mirrors.

FIG. 4 shows the spatial relationship between the incident beam and thereflected beam between the surface and polygon mirror.

FIG. 5 shows the spatial relationship between the incident beam, thereflected beam and the capture mirror.

FIG. 6 shows a representative three lens implementation of thetelecentric lens be assembly.

FIG. 7 shows an alignment aperture mask.

FIG. 8 shows the polygon scanner orientations in a two channelembodiment.

FIG. 9A and 9B show baffles and air knives in a inspectionsubcompartment and 9C shows the lifter supporting the disk.

FIG. 10 shows the relationships between the conveyor, the lifter and thesubhousing.

FIG. 11 illustrates a top view of the disk carriers.

FIG. 12 illustrates a control system for synchronizing and offsettingthe rotation of the polygon scanners.

FIG. 13 illustrates the data acquisition path.

FIG. 14 is a flow chart of the data analysis.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The preferred embodiments of the inventions will be described inrelation to a laser based inspection tool for inspecting the planarsurfaces of disks for use in disk drives. The inspection may beperformed on substrates or finished disks and is preferably performed onboth surfaces of the disk simultaneously. The Laser Inspection Tool(LIT) is general in that it can be used to inspect any sufficientlysmooth flat surface at any stage of the process; therefore, it could beused to inspect raw or initial substrates, substrates afternickel-phosphorous coating, or finished disks. The LIT uses low anglereflected light rather than scattered light from the surface to simplifythe design, to allow absolute reflectivity measurements if desired andto aid in the detection of certain types of disk defects such as stainswhich do not effect the scattering of the light. Stain detection isaccomplished through the use of derivative analysis of the reflectedlight to detect the change in the reflectivity of the surface associatedwith a disk stain. The system is designed to preserve both thepolarization and the wave vector of the reflected light which allows itto be used with minor modifications in a broad range of applications.Using a stable laser, low noise detectors and sufficiently highresolution A/D converters, it is possible to detect a change inreflectivity of approximately 0.1% using the LIT. Since the surfaces ofthe disks are extremely sensitive to physical contact the LIT uses amechanical lifter which, without clamping or spinning, moves the diskthrough the laser scan lines to allow the entire surface on each side ofthe disk to be scanned. Inspection or test systems which require thedisks to spin are complex and increase the risk of damage to the disk.The line scanning is performed using a rotating polygonal mirror(scanner) which also captures the beam reflected from the disk surface.As the disk is lifted into the scanning area it passes through a pair ofair knives which blow loose particles from both surfaces.

FIG. 1 shows the optical path elements for the optical system of onechannel (the A-channel) of the LIT from the laser source 11 to the disk17 and the disk surface to be inspected 17a in a preferred embodiment.The elements in the second channel (the B-channel) for inspecting theother surface 17b are identical, but are preferably arranged in a mirrorimage of the A-channel elements and in the same plane. The A andB-channels can be assembled on a single baseplate. The choice of lowpower laser is not critical, e.g. GaAs, HeNe, etc. are acceptable, butit is preferable that the wavelength be in the visible spectrum foraiding alignment. The use of an unpolarized laser is preferable, sinceit reduces sensitivity to the orientation of defects such as scratches.A few milliwatts of power is sufficient. Since high sensitivity toabsolute amount of reflected light is a goal in the design of the LIT,it is important to select a laser, e.g. HeNe, to minimize noise whichmight be injected into the system through laser instability. In thepreferred embodiment separate lasers are used for each channel, but itis also possible to use a single laser source with a beam splitter.Lenses 12 and 13 form a telescope (beam expander) which is used toexpand the beam 23 (the incident beam). The term incident beam (orA-beam to specify the A-channel) will be used to refer to all segmentsof the laser beam from generation at the laser 11 along the path to thesurface of the disk being inspected (or as will be noted later anoptional calibration mirror). Steering mirror 14 reflects the beam ontothe rotating polygon scanner 15 which reflects into the telecentric lensassembly (TLA) 16. The TLA acts to keep the laser beam incident at aconstant nearly perpendicular angle as the beam scans across the disk.The TLA is designed to have a very flat field curvature through thescanning line to keep the spot size sufficiently constant for accuratedetection. The figure shows the polygon scanner 15 rotating in aclockwise direction which will cause the incident beam to sweep fromleft to right across the TLA and in turn to scan a line across theplanar surface of the disk. Each mirrored facet of the polygoncorresponds to one scan line across the disk. The choice of a polygonscanner is preferred, but other scanning means such as a galvonometermirror could be used. The view of FIG. 1 can be considered to be a topview which shows only the top edge of the disk or other item having theplanar surface to be inspected. The TLA should have a usable opticalscan line which is at least equal to and preferably slightly longer thanthe desired scan length. A laser spot size on the disk of approximately50 microns in diameter provides sufficient resolution for detectingdefects in current disks. Smaller spot size can be used to increase themaximum resolution of the system if desired by altering the focallengths of the telescope lenses.

FIG. 2 shows the optical elements in the path for the reflected beam 24from the surface being inspected. The term reflected beam (or A/R-beam)will be used to refer to all segments of the beam which is reflectedfrom the object's surface as it follows the path to the detector. Thesurface of the object 17 reflects a portion of the incident beam to forma reflected beam 24 which follows a path back through the LIT which isslightly offset from the path of the incident beam. (Note: The describedembodiment inspects the planar surfaces of disks, but nonplanar surfacescould be inspected using the system if the nonplanarity is no more thana few degrees.) The reflected beam passes through the TLA 16 and isreflected by the scanner 15 back to mirror 14. Because the path of thereflected beam is offset from the incident beam the reflected beamstrikes capture mirror 18 which diverts the reflected beam through lens20 which reduces the spot size of the beam striking detector 19. Thedetector is preferably a silicon detector which produces an analogsignal which is a function of the amplitude of the reflected beam. Thedetector should have very low noise to preserve the sensitivity of thesystem. The LIT may function by detecting only relative shifts in thereflected beam as it scans across the surface and as the surface istranslated under the beam, but it is advantageous to detect absolutereflectivity. The use of reflected light for inspection rather thanscattered light allows a simplified approach and avoids the problemsinvolved in trying to capture all of the scattered light. In additionthe use of reflected light allows detection of absorption changes anddefects associated therewith.

FIG. 3 shows an optional feature which provides a start of scan signaland allows the detector to be calibrated to measure the absoluteamplitude of the reflected beam. Measurement of the absolutereflectivity allows an additional class of defects and/orcharacteristics to be detected and/or measured, thus enhancing thecapability of the tool. Calibration mirrors 21 and 22 are arranged sothat an initial portion of the scan line falls on mirror 21 whichreflects the beam to mirror 22 which reflects the beam back to mirror 21and back into the TLA along the path for the reflected beam as describedabove. The length of the path of the beams going to and from thecalibration mirrors is set equal to the length of the beam paths to andfrom the surface 17 to prevent spot size change. This arrangementcreates a reference signal from the detector for each scan line whichsignals the start of the scan and is also known to correspond to maximumpossible magnitude of the reflected beam. Alternatively a mirror couldbe positioned adjacent to the object being scanned to allow the beam tostrike the mirror during the scan, but positioning the calibrationmirrors away from the disk as shown in FIG. 3 is preferable since itreduces the number of fragile components near the mechanical movingparts. Having the maximum reference signal for comparison allows theamplitude of the reflected beam from the disk to be converted to anabsolute measure of reflectivity. The signal from the calibrationmirrors can be used as a start of scan without using it as an absoluteamplitude reference. Once the beam strikes the Calibration Mirror 21 thereflected beam will slew to its maximum value. This transition from noreflected beam to the maximum forms a sharp edge in the analog output ofthe detector which can be used as the start of scan signal. A fixeddelay can then be used to gauge the approximate time at which the scanline will be at the first data point on the disk. To avoid having falsetriggering from the other transitions in the signal at the edges of thedisk, the circuitry which detects the start of scan signal should delayresetting until the scan line has cleared the last edge of the disk.

FIG. 4 shows the preferred spatial relationship between the incidentbeam and the reflected beam between the surface 17 and polygon mirror15. As previously noted the reflected beam 24 is offset from theincident beam 23 to allow the reflected beam to be routed to thedetector. This is achieved by causing the incident beam to strike thesurface at a slight angle which causes the reflected beam to come off ata slight angle as shown in the FIG. 4. As an example, an offset angle ofa few degrees over a 125 mm path results in a beam offset of on theorder of 5-10 mm which easily allows the reflected beam to be routed toa mirror which is by-passed by the incident beam. The telecentric aspectof the TLA causes the reflected beam 24 to be essentially parallel toincident beam 23 after the reflected beam has passed through the TLA.The optical axis of the TLA should ideally split the angle formed by theincident and reflected beam at the surface to minimize the effects ofcoma and spherical aberration due to the beam separation. FIG. 5 showsthe spatial relationship between the incident beam and the reflectedbeam in relation to the Capture Mirror 18 and Steering Mirror 14. Theincident beam 23 passes above the Capture Mirror 18 on its way toSteering Mirror 14. The reflected beam 24 is sufficiently offset toallow it to strike Capture Mirror 18 and to be routed to the detector.This arrangement is deemed superior to using a beam splitter with thesignal losses associated therewith. It is feasible to allow the incidentand reflected beam to be coincident until the reflected can be separatedusing an appropriate beam splitter, but the arrangement shown is deemedpreferable. Beam splitters which could be used if desired includepolarizing beam splitters, partially reflective beam splitters, orpellicle beam splitters.

The TLA's characteristics are tailored to the specifics of theapplication and particularly to the size of the surface being inspected.One standard size of disk for use in disk drives is 95 mm in diameter.For such a surface the design of the TLA could be specified for thewavelength of the laser being used as a field of 105 mm, focal length125 mm, telecentricity<0.5 degrees and field curvature of<1.0 mm. FIG. 6illustrates a three element spherical lens implementation which can beused to meet these requirements. Other implementations (including asingle lens) may be used. An optical configuration which is capable ofscanning 95 mm disks is also capable of being used to scan smallerdisks. When smaller objects are being scanned it may be desirable toincrease the sampling rate in order to obtain the same number of pixelsfor the smaller object.

FIG. 7 shows an optional aperture mask 71 having apertures (holes) 71aand 71b which is positioned between scanner 15 and mirror 14 so that theincident beam 23 passes through aperture 71a and reflected beam 24passes through aperture 71b. The aperture mask is made from opaquematerial and the apertures are sized to limit the cone of scatteredlight that passes through. Limiting the cone of the reflected beam ismore important than for the incident beam. The reflected beam cone sizeshould be limited to removing some of the near forward scattered lightto improve the sensitivity of the detector to reflected light. The sizeof the aperture for the reflected light should be selected empiricallyfor the particular application by scanning a surface feature known toproduce near forward scattered light, e.g., laser texture bumps ondisks. The aperture should not be overly restrictive, however, sincevariation in the polygon faces, etc. will cause some tolerances to existin the reflected beam. Using an aperture of approximately six times thediameter of the beam in the arrangement shown was found to result in asuperior detection capability for the laser texture zone. The masking ofthe near forward scattered light can be performed at various points inthe path of the reflected beam, e.g. at the detector, but the positionof the aperture mask as shown has the additional benefit of providing anaid for coarse alignment of the beams which should be positioned in thecenter of the apertures. Optionally, a removable translucent member maybe placed over the apertures to enhance the visibility of the beamposition during alignment.

Various other alignment features and aids may be incorporated into thedesign of the LIT. For use in a manufacturing environment it isimportant that alignment be easy to obtain and to maintain. In a twochannel implementation the disk must be centered between the two TLAsand be perpendicular to the horizontal axis of the system. The positionof the disk holder with respect to the optics board is adjusted, thenthe distance position along mounting rail of the TLAs is adjusted.Preferably several of the components in the optical path will have x-yadjustments, but it is not required that they all have adjustments. Thelenses 12, 13 comprising the telescope are likewise mounted on rails toallow the distance between them to be adjusted to control spot size. Atleast one of the lenses in the telescope should preferably have a fineposition adjustment.

The apertures in the aperture mask may also be used during the fineralignment process by positioning target plugs in the apertures whichhave small diameter alignment holes, e.g. 1 mm in diameter, positionedat the central point where the beams are properly aligned. One or moresimilar targets with alignment holes may be used in the path from thelaser to the steering mirror. If the laser and the telescope lenses aremounted on an optical rail, one or more targets with alignment holes canbe placed on the rail and moved along the rail if desired to aid in thex-y alignment of the laser. The approximate alignment of the beams canbe observed visually since misalignment will reduce the intensity of thebeam passing through the small holes. After coarse alignment has beenachieved, the amplitude of the beam as measured by the detector 19provides a precise aid for alignment of the entire path. The x-yadjustments of the laser, the steering mirror, etc. are used to achievemaximum amplitude of the output signal at the detector from a referencedisk having at least a portion which is used as a defect free standard.

The TLA determines the spot focusing and telecentricity of the beam.Telecentricity is determined by the spacing of the TLA to the polygonand is set using the scan lens micrometer which translates the TLA alongthe axis between the polygon and the disk. The input beam collimation ofthe incident beam entering the TLA is set by adjusting the spacingbetween lenses 12 and 13 of the telescope. The circularity of the returnbeam after reflection from the polygon is a function of thetelecentricity and can be used during alignment. Spot size should bereasonably constant through a scan, so spot size should be measuredacross the scan field using a spot size measuring instrument positionedin the scan plane at multiple scan positions. If the deviation in spotsize is too large, additional alignment is needed.

Calibration Disks

A calibration disk or disks with precise laser marks may also be used tosimulate scratches to further calibrate the tool so that the various LITtools will provide repeatable and comparable results. For example, acircular "scratch" of precise width (i.e., a fixed number of pixels) isformed by laser into a set of calibration disks then each system can beadjusted to detect the correct number of pixel defects. This isaccomplished by producing a standard, laser-produced feature on areference disk. The feature is a ring produced by continuous exposure ofa focused CO2 laser onto a glass disk. The glass disk is vacuum mountedonto a spindle which is rotated under the focused beam. By adjusting theexposure parameters (laser power, spindle rpm and exposure duration) theheight and width of the ring cross-section is adjusted to produce asignal level in the range of the defects of interest. A typical set ofparameters is approximately 50 mW laser power, 15 rpm spindle speed and1 second exposure. The laser-exposed glass disk is then coated with astandard metallic/magnetic coating of a thin film disk of the type to beinspected. This will produce the same reflectivity and thus, opticalsignal as is obtained when inspecting the actual disk product.

For calibration, the calibration disk is scanned by the LIT using aspecial calibration program. This program has a mask adjusted to measurethe laser-produced ring inside a standard annulus. The defect pixelswithin this annulus are measured and compared to the desired value. Thiscalibration technique incorporates a simple way to determine inspectiontool variation. Any variation in tool parameters affecting the normalpixel count such as optical alignment, laser power drift, liftervariation, mechanical stability, etc. will be detected in the scanningof the calibration disk. This serves as a simple means to verifyconstant performance of the surface inspection tool.

The tooling for producing laser zone texture on glass disks can be usedto produce the calibration disk. The formation of laser zone texture onNiP/Al:Mg disks and glass disks both use a pulsed laser to produce aspiral array of microscopic texture bumps in a well defined radial zone.However, because the absorption wavelength of glass is significantlydifferent from NiP-coated metal disks, the glass texture processutilizes a carbon dioxide laser (CO2). For the laser texture process forglass, a specially designed CO2 laser is used which emits radiation at anon-standard 9.25 microns wavelength. This wavelength is specified sincethe aluminosilicate glass disks utilized for drive manufacturing havetheir peak absorption near this wavelength. The laser texture tool forglass utilizes a temperature-stabilized, continuous-wave, CO2 laserwhich is modulated (i.e. pulsed) by an acousto-optic modulator (AOM).Two identical beams are emitted from the AOM which form the tworadiation channels for performing simultaneous double-sided texturing ofa disk. The AOM is used to control both the beam intensity, and thepulse duration. The pulsed laser light is passed through a focussinglens prior to impinging on the disk in a spot size that can vary from 30to 100 microns. The pulse duration can be varied between 1 and 10microseconds. Each laser pulse produces 1 texture bump. The radialtexture zone is produced by simultaneously spinning and translating thehub-mounted disk.

Since the glass texture tool is capable of producing "defects" (i.e.texture bumps) on smooth glass surfaces, it was reasoned that thisdefect could repeatably and controllably be placed on a glass disk.Further, if the defect was found to provide a sufficient and repeatablesignal for the LIT process, then the glass texture tool could providemeans for fabricating calibration disks. To create the calibration disk,the disk was rotated, but not translated, during the laser exposure.This produces a circular defect whose radial width was approximately 10to 20 microns. Various pulse durations and pulse repetition rates wereinvestigated, ranging from continuous-wave "scribing," to formingperforated marks by utilizing 16 millisecond pulses at 25% duty cycle,to forming defects by using 1 microsecond pulses at 20 kHz repetitionrate. The LIT signal level could be increased by forming the circulardefects at higher laser intensity levels, by utilizing longer exposureperiods for a fixed intensity, or by reducing the disk spin rate. Aftertesting many disks, a "best mode" procedure for defect production wasdetermined. The disk onto which the defects are to be fabricated ismounted on the hub and spun at a rate of 15 revolutions per second. Byvarying the radial position of where the laser light strikes the disk,say from 20 mm to 30 mm, the LIT signal level from the fabricated defectcan be made to decrease. By using a shutter in the laser beam path,precise 1 second exposures are produced on the disk. The laser is pulsedat 20 kHz, and the pulse duration is 1 microsecond. The average laserpower incident on the disk varies between 50 and 100 milliwatts,depending upon the desired signal level of the produced defect. Inoperation, once the shutter is opened, the laser beam is passed througha beam expander to increase its size to approximately 7 mm diameter. Theexpanded beam is then passed through a 25 mm focal length lens prior tobeing focussed onto the disk. The estimated focal spot size is 30microns which produces a bump diameter of about 10-20 microns. Becauseof the gaussian nature of the focussed beam, the produced defect has awidth less than the focal spot size. The range for the diameter of thebumps should be about 10 to 40 microns which also defines the width ofthe circular feature. A typical range for the height of laser formedbumps is on the order of 15-50 nm. The exact dimensions (width andheight) of the defect depend on parameters such as intensity level, spotsize and linear velocity of the spinning disk. At 1 sec. of exposure,and a disk spin rate of 15 rps, 15 revolutions of the disk are made pastthe focussed laser spot. After this time, the shutter is closed and theprocess terminated. Since each pulse is producing a discrete texturebump (as described above), the fabricated circular defect consists ofapproximately 20,000 such bumps circumferentially overlapped around thedisk, and all at the same radius.

Polygon Synchronization

In a two channel implementation of the invention, i.e., a LIT whichinspects both planar surfaces of a disk simultaneously, there areseveral options on how the A and B-beams can be spaced and coordinatedwith each other. Interference between the channels will occur when thebeams pass through the central hole in the disk unless steps are takento prevent it. A similar problem might arise if the LIT is being used toinspect any object having holes and/or transparent portions. Thepolygons can be arranged to rotate in a common direction which willresult in the scan on each side of the disk proceeding in oppositedirections and crossing in the center of the scan area. When the beamscross there will be crosstalk as the beam from the opposite channelreaches the detectors. One arrangement to minimize interference has theB-beam vertically displaced from the A-beam, but this causesdifficulties in building the tool and coordinating the data. A preferredarrangement is shown in FIG. 8. FIG. 8 shows the polygon scannerorientations in a two channel embodiment where the polygons 15, 15' arecoplanar, but spin in opposite directions which results in the two beams23, 23' maintaining a fixed relationship to each other as they scan. Ina preferred embodiment the rotational position of the two polygonalscanners are synchronized, but with one being advanced or retarded intime to avoid interference when the beams scan across the hole in thecenter of the disk. The beams will still pass through the hole in thedisk and be reflected off of the opposite channels polygon, but will besufficiently separated from the path of the reflected beam to bediverted away from the detector. FIG. 12 illustrates a control systemfor synchronization and offset of the polygons. Beams 23, 23' are shownpassing through the hole in the disk. The beams are only shown up to theTLAs 16, 16' but each will of course pass through the transparent lensesand be reflected from the polygon facet. The separation between thebeams is shown greatly exaggerated so that the offset is clearlyvisible. The polygons are rotated by DC motors 110, 110' which haveintegral position sensing units 111, 111' which can provide index pulsesas well as a binary value indicating the angular position of the motor.The positional signals are fed back into the Synchronizer/Clocking unit113. This provides signals to the motor drivers 112, 112' which controlthe speed and phase of the motors. Using the position feedback and anexternal input, the Synchronizer/Clocking unit retards or advances thephase of one of the motors (e.g. motor 110') to allow magnitude of theoffset between the two motors to be controlled while maintaining thesame rotational speed. The external input signal can be as simple aspulse generated by an operator pressing a switch which causes the phaseto advance a few microseconds for each pulse. One convenient way to setthe offset is to adjust for zero crosstalk. An operator could do this byobserving an oscilloscope trace of the output of one detector andbumping the phase switch until the signal is essentially zero for thehole area of the disk.

Mechanical System

FIGS. 9A and 9B show air baffles and air knives in an optionalinspection subcompartment which can be used in the LIT. FIG. 9A is across-sectional side view of the disk 17 being supported by the lifter91 between the air baffles 92 and 92'. The frontal view of the disk andlifter is illustrated in FIG. 9C and the air baffle structure is shownin FIG. 9B. Each of the baffles has a beam port 93 which is a horizontalopening in the baffle through which beams 23 and 23' pass to strike thedisk. The beam ports also provide a path for the air flow out of theinspection subcompartment. As shown the lifter 91 is a paddle-likemember with a longitudinal slot in the top which engages the edge tosupport the disk in the vertical position without clamping. The liftercan be of any shape and design which allows the disk to be held withoutdamage and moved through the scan line. For example, the body portion ofthe lifter could be a rod. The disk is supported from the bottom to usethe weight of the disk to retain the disk in the slot and help eliminateany need for clamping. The cross section of the slot preferably hasV-shaped sides and is curved to match the curvature of the disk. Asshown the top of the lifter is rectangular, but alternatively can beV-shaped or curved to further minimize the amount of the disk surfacewhich is obscured. The top of the lifter may also have fingers to holdthe disk. The slot end portion of the lifter should be designed toobscure no more of the disk than is necessary to support the disk sinceit limits the available inspection area. A disk for use in a disk drivehas a small band around the outer perimeter which is not used forrecording which allows a small area to be obscured for support withoutreducing the effectiveness of the inspection. Although the lifterarrangement is considered preferable, any alternative mechanism(s) whichprovide the function of moving the disk into the inspection area andthrough the scan lines are feasible. The movement of the disk should beorthogonal to the scan lines, therefore, in the described embodiment thescan lines are horizontal and the movement of the disk is vertical, butif the scan lines are vertical, then the movement should be horizontal.Other examples of means for moving the disks in and out through the scanlines include a conveyor system or any type of mechanical arm. Thecurved v-slot can be used to support the disk in combination with anyappropriate movement system, but it is also possible to grip the disksby the edges.

The preferred embodiment is shown as a two channel system which inspectsboth sides of the disk simultaneously. Alternatively, a single channelsystem could be used to inspect both sides of the disks either by havingthe mechanical system flip the disk around and repeat the scan on thesecond side of the disk or by splitting the scanning beam and usingadditional mirrors to direct the split scanning beam to the second sideof the disk.

The air knives 94, 94' are used to direct a shaped stream of partiallyionized air onto each surface of the disk as it moves into thesubcompartment. The air knives are nozzles which form a fan shaped airflow pattern which strikes the disk surface in a cross section ofrelatively narrow height, but with a width extending across the diameterof the disk. The action of the air streams is to blow off looseparticles from the surfaces which might otherwise be detected as adefect in the disk. If the scan is performed when the disk is movingdownward, then the entire disk will have passed through the air knivesprior to the start of the scan. It is preferable to turn the air knivesoff when the scan is actually being performed to minimize vibrationswhich might introduce noise into the data. It is also preferable thatthe air knives provide approximately equal force on the surfaces of thedisk so that the net force on the disk is minimal and the disk can besupported without clamping. Preferably the air coming into thesubcompartment will be relatively clean. In addition to optionallyoperating the LIT in a clean room environment, additional filtering maybe advantageous. For example, a HEPA filter may be convenientlyinstalled above the subcompartment so that filtered air can be forcedinto the top of the subcompartment and out of the beam ports in the airbaffles. There will also be a slight air flow out of the bottom of thesubcompartment through the entrance port used to lift the disk into thesubcompartment to help reduce contamination from other parts of theapparatus.

Automatic Feeding System

The following section describes an embodiment of a system for automaticfeeding of disks into the inspection area. Disks could be placedmanually or by robot arm in the lifter, but for high volumes it isclearly preferable to have a simple but efficient automatic feedingsystem similar to one described below. Operators feed the tool byplacing groups of disks in plastic carriers 105a-c on a conveyor belt106. The carriers move along the conveyor belt underneath the actuallaser apparatus until they reach the loading point where a lifter picksthe disks up out of the carrier and pushes it up into the path of theair knives and the scan lines. FIG. 10 shows the relationships betweenthe conveyor 106, the lifter 91 and the subcompartment 101. The disks 17are brought into position over the lifter by being moved in diskcarriers 105a-c placed on the conveyor system 106. When a disk is inposition the lifter extends up through the conveyor, through an opening107 in the bottom of the disk carrier to engage the disk 17. FIG. 11shows the top view of a disk carrier 105. The slot 91a in the top of thelifter engages the bottom edge of the disk and lifts it out of thecarrier up into the subcompartment 101 where the scan occurs. Theextension range of the lifter must therefore be large enough to movefrom its initial position below the conveyor to a point at or above thescan line which will probably be equal to several times the diameter ofthe disk. The speed at which the lifter moves the disk through the scanline is a parameter which needs to be coordinated with the scan rate andthe signal processing. Once established the movement rate should notvary.

The conveyor 106 must have sufficient sections and controls to positiona disk carrier so that the first disk is over the lifter, then pausewhile the disk is lifted out and returned. If there are multiple disksin the carrier, the conveyor must index to the next disk and repeat theprocess for each disk in the carrier. The conveyor functions may becontrolled by the same computer which processes the scanned data, butthe functions are sufficiently simple to allow use of a standardprogrammable controller. When one carrier is finished it must be movedto an unloading area. Since the lifter must extend up through theconveyor, the conveyor must have openings which are always under thedisk at the lift point. This could be done with spaced slats, cutouts,louvers, removable slats, a bulldozer type dual track system with a gapbetween the tracks, etc.

The disk carriers are shown with three disks, but could be designed forany number of disks including one. The requirements are that the disksbe held sufficiently upright to engage the slot in the lifter and, ofcourse, that there be clearance for the lifter to enter through thebottom to lift the disk out of the carrier. The curved v-slots asdescribed for use in the lifter can also be used in the carriers with aslot on each side of the carrier to support each disk. The carriers canconveniently be made of plastic.

Data Capture and Analysis

FIG. 13 illustrates a possible data acquisition path for one channel ofan LIT. The analog signal from the detector 19 is optionally filtered byfilter 121 before being sampled and digitized by the A/D converter 122to get a value for the reflectivity at the sample points. The samplingrate should be selected in coordination with the time required for eachscan line and the desired resolution. For example, if the usable portionof each scan line requires 1 ms and a resolution of 2000 pixels per lineis wanted, then a sampling rate of 2 MHz is needed. The spacing betweenthe scan lines is the distance that the disk moves vertically during onescan. If 2000 vertical pixels are desired for a 95 mm disk, then thevertical movement should be about 47.5 microns per scan. Each digitalsample value represents one pixel in the image of the surface of thedisk. The data acquisition hardware 123 places the pixel data in abuffer which is accessible by the computer 124. Since the surface isscanned in lines, the pixel data is organized into lines as well. Sincedata acquisition is a common requirement, there are commerciallyavailable cards which can be plugged into slots in a PC, e.g., PCl busslots, which will perform the required function at a sufficiently highrate. The A/D unit may also be included on such a card. The computerwhich processes that data can be any general purpose computer orworkstation which has sufficient speed to process the data in the timeallowed, e.g., within a few seconds. The data acquisition path for thedata from the second channel, if present, should be identical to the oneshown. The computer can process the data for the two channels if it isfast enough, but it is also feasible to use separate computers for eachchannel. If separate computers are used, the results can be communicatedfrom the B-channel computer to the A-channel computer which can then actupon the consolidated results by rejecting the disk or reporting theresults through the network communication facilities to the master floorcontrol system. At least one computer in the system should have adisplay 125 on which the enhanced image(s) can be displayed as well asmessages, etc. Similarly at least one keyboard 126 should be availableto allow parameter entry, manual control, maintenance, etc.

If the optional calibration mirrors are used, then the first portion ofeach scan line will correspond to maximum reflection and can be used asa reference signal for finding absolute reflectivity and as a start ofscan signal for the hardware. The abrupt signal change which correspondsto the edges of the disk can be used to find the position of the edgesin the line of data. For lines which cross the central hole in the diskthere are four edges, otherwise there are two edges per line. Since itis not desirable to test 100 percent of the surfaces of the disks,provision has been made to exclude portions of each line by use of amask. This could be done on a pixel by pixel basis with a flag bit foreach pixel indicating whether it should be processed or not, but thismethod requires a relatively large amount of storage. A preferableapproach is to record start and stop points in a table for each scanline. Using four numbers per line, e.g. x1, y1, x2, y2 allows thesoftware to exclude the first x1 pixels in the line on the disk fromprocessing, then process all pixels until pixel y1 is reached at whichpoint processing is suspended again until pixel x2 and then continuesuntil pixel y2 where processing of the line stops. The set of thesenumbers corresponding to the shape of the image of the disk beingscanned will be called the mask. Since the mask is referenced to theedge of the disk, it must be located on pixel data in the buffer. Theimage of the disk in buffer does not always appear at the same place forvarious machines, times, etc. and, therefore, there is a need to fit themask to the particular data by finding (or predicting) the location ofthe edges. Note that even though the disks are circular it is possiblethat the image of the disk may be elliptical due to artifacts of thesystem. For a line with only two edges and no laser bump texture onlytwo numbers are required in the mask, since there is no need to skipover an area in the center of the disk, but it may be convenient to havefour numbers for each line for simplicity. If an object other than adisk with a single central hole were to be scanned by the system, themask could be adjusted accordingly by adding or removing numbers tocover the maximum number of the starts and stops required.

FIG. 14 is a flow chart for the data analysis. Initially the data isprocessed one line at a time, so the pixel data is read line-by-line 131and the line process is repeated until the last line 138. The edge ofthe disk in the line is found 132. The mask is applied to the linestarting with the edge 133. The unmasked portions of each scan line areprocessed for defects by use of median filter 134. A selected number ofpixels (e.g. 30-150) in a sliding window are averaged. A threshold aboveand below this running median is used to define a defect pixel. Thisthreshold should be selected empirically at a level which mostaccurately finds true defects. A deviation of only a few percent (e.g.3-4%) might be appropriate for a very uniform surface of a disk.However, single defect pixels are not generally significant for disksand may only be noise, so it is desirable to define an additional filterwhich looks for groups of defect pixels. Requiring that a predeterminednumber (e.g. 5-10) of consecutive defect pixels might be a simple way toachieve this, but could be sensitive to signal to noise problems. It ispreferable that consecutive pixels not be required. A preferableanalysis looks at the total number of defects in a selected region whichcan be a sliding window. The appropriate parameters for the window sizeand percentage of defect pixels which are tolerable should be determinedempirically based on the particular application. The selected parametersshould be programmable by operator input, but, for example, setting thethreshold to 6 defect pixels out of a 3×3 pixel block might be typicalfor a magnetic disk scanning application. It has been found, however,that as the signal to noise ratio of the system increases, the need fora regional determination decreases. Another way to look at the data isto compute the maximum defect region size for the entire surface andcompare this size to a selected threshold. Any standard statisticalanalysis of a pixel group of any size up to the entire surface whichdetermines the probability that a particular deviation is due to chancecan be used. A practical application of the LIT will probably usemultiple tests of the sort described to define any number of defectsignatures in the pixel data. Because the thresholds may need to bechanged and experiments done to find the optimum value, they should bemade programmable by operator input.

One optional method for looking for defects is to find the rate ofchange in the reflectivity using the derivative of the pixel stream. Ithas been discovered that a particular type of disk defect called a stainis not detectable using the median filter described above, but isdetectable using the derivative because they are associated with a rapidchange in reflectivity from one level to another 136. The second levelof reflectivity persists for a relatively large distance so it is notseen as a defect using the median filter. Thresholds for the rate ofchange to be detected as a defect should likewise be defined empiricallyand be programmable.

Some, but not all, disks have a circular band a few millimeters aroundthe hole which has been intentionally roughened (textured) by creationof thousands of small bumps to form a contact start/stop zone for aslider in a disk drive to physically contact whenever the disk is notspinning fast enough to fly the slider. Since these bumps are largeenough (tens of nanometers) to be considered defects if located in thedata area of the disk, the textured area must either be excluded fromprocessing or processed using different parameters from those applied tothe data area. The texture area can be detected by the presence of asmall but sharp decrease in the reflectivity of the surface. This sharptransition is found by calculating the spatial derivative everywhere inthe expected region for the texture and is then sorted to find themaximum and minimum values. It may also be useful to average severallines or areas of pixels in order to effectively increase the signal tonoise ratio of the data. The disk qualifies based on the position of thetexture area being where it should be and the change in reflectivitybeing within a selected range. The change in reflectivity can becorrelated to bump geometry and provides a way to confirm thatmicrotopography of the texture is correct 140.

For highest processing speed or greatest throughput, the dataacquisition process and data analysis can be performed simultaneously orconcurrently. In order to accomplish this, the data analysis must a)check for valid data and b) predict mask location in the buffer, beforeall edges have been discovered. The presence of valid data in the buffercan be ascertained by querying the data acquisition electronics foramount of data transferred (if that capability exists as it does in somecommercially available systems) or by timing. Prediction of masklocation is performed with the knowledge that the disk shape does notchange from disk to disk and that only small changes in disk positionwill occur. Given this, the edges discovered in the first 10% of thedisk image are used to adjust the mask positions within certain limits.The analysis of the data in the buffer can then proceed even though thescan of the disk is not complete.

A disk that is flagged as defective can be handled in various waysincluding simple reporting, displaying the image, calling an operator orrejecting the disk 141. It is, of course, possible to build thecapability into the LIT to sort the defective disks from the passingdisks. It is also feasible to simply record the test results by carrierserial number and disk position and place the defective disks back intothe carrier along with the passing disks. The results data can beelectronically communicated to a shop floor control system or some formof printout or other marker might be attached to the carrier. The datacould then be used to sort disks at a later time. One advantage ofdelaying the actual sorting of the disks to a subsequent time is that itallows the final decision to take additional tests into account and,thereby increases the flexibility of the system.

Another optional test that can be applied to the pixel from a finisheddisk which has sputtered thin films (or similar surfaces) is to select aset of areas distributed over the surface which are composed of multiplepixels and determine the average reflectivity of each area 139. If theaverage reflectivity of the selected areas varies more than a selectedthreshold, it may indicate a failure of the sputtering process touniformly coat the disk. This type of test must be tuned empirically tothe specifics of the surface being inspected and the process by whichthe surface is created.

One alternative test that can be applied using the alternativeembodiment of the LIT which provides for absolute measurement ofreflectivity involves estimating the thickness of the hydrogenatedcarbon (C:Hx) overcoat which is typically sputtered onto thin filmdisks. Table 1 shows the relationship between reflectivity and thicknessof a typical C:Hx coating used on a thin film magnetic disk with acobalt alloy magnetic layer. Although the hydrogen content of a C:Hxfilm can vary depending on the deposition process and parameters, atable of reflectivities can be established for any particularcomposition being used and an acceptable range can be defined for thetest. Using the Table 1 data, if the established range for an acceptableC:Hx overcoat for a particular disk type is 100-200 angstroms, then thereflectivity measured by the LIT should be between 0.51 and 0.575 forthe disk to pass inspection. This method can obviously be extended toany other surface coatings whose thickness can be mapped toreflectivity.

                  TABLE 1                                                         ______________________________________                                        Reflectivity                                                                              C:Hx Thickness (Angstroms)                                        ______________________________________                                        0.64        0                                                                 0.625                    50                                                   0.575                    100                                                  0.56                      150                                                 0.51                      200                                                 0.46                      250                                                 0.4                        300                                                ______________________________________                                    

The LIT system described herein is capable of detecting defects withouthuman assistance, but an optional feature provides a display of theenhanced image(s) of the disk surface(s). There are a virtuallyunlimited number of ways that the pixel and defect data can be displayedin the form of an image. Obviously the magnitude of the pixel data canbe converted to display intensities and/or colors. The defect pixels andareas should be displayed in a distinguishable manner, e.g. red dots ona gray background. If the derivative option is being used, then the mapof the derivative value could be displayed rather than or in addition tothe absolute value of the pixels. The mask could also be displayed, aswell as the identified textured area. Display of the data is consideredto be a very powerful option which allows human pattern recognition tosupplement and monitor the functioning of the tool when desired.

The various inventions described herein have been illustrated in theirpreferred embodiments, but variations within the scope of the inventionswill be apparent to those of skill in the art.

what is claimed is:
 1. An apparatus for scanning two parallel planarsurfaces of an object using a first laser beam (A-beam) having a firstpath and a second laser beam (B-beam) having a second path, comprising:asubcompartment having at least one opening; first and second air knivesin the subcompartment with the first air knife positioned to directpartially ionized air on the first parallel planar surface and thesecond air knife positioned to direct partially ionized air on thesecond parallel planar surface; and a movable support for the objectwhich holds the object with first and second parallel planar surfacessubstantially perpendicular to the A-beam and the B-beam and moves theobject into the subcompartment between first and second air knives andthrough the A-beam and the B-beam with the A-beam striking the firstparallel planar surface and the B-beam striking the second parallelplanar surface.
 2. The apparatus of claim 1 whereinthe movable supportmoves upward by the first and second air knives and then downward out ofthe subcompartment; and means for turning the first and second airknives off before the movable support begins to move downward.
 3. Theapparatus of claim 2 wherein a force exerted on the object by the firstair knife is substantially equal to and opposed to a force exerted onthe object by the second air knife.
 4. The apparatus of claim 2 furthercomprising means for capturing scan data when the movable support ismoving downward.
 5. The apparatus of claim 1 further comprising:a firsttelecentric lens assembly in the first path which causes the A-beam tobe incident on the first planar surface of the object at a substantiallyconstant angle; and a second telecentric lens assembly in the secondpath which causes the B-beam to be incident on the second planar surfaceof the object at a substantially constant angle.
 6. The apparatus ofclaim 5 further comprising:a first specular light detector whichproduces an analog signal proportional to intensity of light reflectedfrom the first planar surface; and a second specular light detectorwhich produces an analog signal proportional to intensity of lightreflected from the second planar surface.
 7. The apparatus of claim 6further comprising means for sampling and digitizing the analog signalsproduced by first and second specular light detectors into pixel datastored in one or more buffers.
 8. The apparatus of claim 7 furthercomprising data analysis means for statistically processing the pixeldata using selected thresholds for defects and determining whether thepixel data corresponds to one or more defects.
 9. The apparatus of claim6 further comprising a first capture mirror arranged to direct lightreflected from the first planar surface toward the first specular lightdetector, being arranged out of the first path so that the A-beam doesnot strike the first capture mirror.
 10. The apparatus of claim 1further comprising a forced air source which forces air to flow out ofthe opening in the subcompartment.
 11. The apparatus of claim 6 furthercomprising a first beam expander arranged in the first path to increasethe size of the A-beam.
 12. The apparatus of claim 6 further comprisinga mirror arranged to reflect an initial or terminal portion of theA-beam to generate a reference signal.
 13. The apparatus of claim 6further comprising a first calibration mirror arranged to reflect theA-beam during part of the scan to a second calibration mirror which isarranged to reflect the beam back to the first calibration mirror,through the telecentric lens assembly and along the second path to thedetector, the distance from the telecentric lens assembly to the firstcalibration mirror added to the distance from the first calibrationmirror to the second calibration mirror being substantially the same asthe distance from the telecentric lens assembly to the object.
 14. Amethod for scanning two parallel planar surfaces of an object using afirst laser beam (A-beam) having a first path and a second laser beam(B-beam) having a second path, comprising the steps of:moving the objectinto a subcompartment having at least one opening between first andsecond air knives to direct partially ionized air onto first and secondplanar surfaces; moving the object through the A-beam and the B-beamwhile holding the object with the first and second planar surfacessubstantially perpendicular to the A-beam and the B-beam; and samplingand digitizing reflected light from the first and second surfaces toobtain pixel data in one or more buffers while the object is movingthrough the A-beam and the B-beam.
 15. The method of claim 14 whereinthe step of moving the object between first and second air knives movesthe object in a first direction and the step of moving the objectthrough the A-beam and the B-beam moves the object in a seconddirection.
 16. The method of claim 14 further comprising the step ofturning off the air knives prior to the step of sampling and digitizingthe reflected light.
 17. The method of claim 14 further comprising thesteps of:rotating a first polygon scanner in the first path about anaxis to direct the A-beam through a first telecentric lens assembly toscan the A-beam across the first planar surface of the object at asubstantially constant angle; and rotating a second polygon scanner inthe second path about an axis to direct the B-beam through a secondtelecentric lens assembly to scan the B-beam across the second planarsurface of the object at a substantially constant angle.
 18. The methodof claim 17 further comprising the steps of synchronizing first andsecond polygon scanners and offsetting an angular position of the secondpolygon scanner from an angular position of the first polygon scanner.19. The method of claim 14 further comprising the steps of analyzing thepixel data using a median filter and determining groups of pixels with adeviation more than a selected threshold.
 20. The method of claim 14further comprising the steps of analyzing the pixel data usingderivatives to determine groups of pixels with a rate of change greaterthan a selected threshold.